function add(a,b){
    return a+b;
}
function jian(a,b){
    return a-b;
}
function cheng(a,b){
    return a*b;
}
function chu(a,b){
    return a/b;
}
module.exports.add = add;
module.exports.jian = jian;
module.exports.cheng = cheng;
module.exports.chu = chu;